With particular reference to the specification and the magnetic properties of materials the following terms are defined below:
Magnetic domain is a region within a material in which magnetic fields of atoms are grouped together and aligned. This means that the individual moments of the atoms are aligned with one another within the region and exhibit magnetization.
Ferromagnetic material is a material in which large numbers of magnetic domains are present. In an unmagnetized condition the magnetic domains in a ferromagnetic material are randomly oriented and the magnetic field strength of the piece of material is zero. When some of the magnetic domains within the material are aligned the ferromagnetic material becomes magnetized. As more domains become aligned the magnetic field of the material becomes stronger. When almost all the domains are in alignment the material is said to be magnetically saturated. When a material is magnetically saturated, additional amount of external magnetization force will cause a negligible increase in its magnetic field strength.
Magnetic field is a change in energy within a volume of space surrounding a magnet. A magnetic field consists of magnetic lines of force surrounding the magnet or an electrical conductor carrying current.
Magnetic flux is the total number of magnetic lines of force in a magnetic field.
Magnetic flux density is the number of magnetic lines of force cutting through a plane of a given area at right angles.
Permeability is the property of a material that describes the ease with which a magnetic flux is established in a piece of that material when an external magnetizing force is applied.
Current limiting in an electric circuit is the process of preventing current in excess of rated current from flowing in the circuit. Current limiting protects the equipment and the circuit wiring from damage caused by flow of high current.
Current Limiter is a device that when included in a circuit effectively limits the current flow within permissible limits.
Current limiting is used in electrical or electronic circuits for imposing an upper limit on the current that may be delivered to a load. This current limiting is carried out to protect the circuit from harmful effects due to short-circuit or similar problems in the load. This term is also used to describe the ability of an over current protective device like a fuse or a circuit breaker to reduce the peak current that flows in a circuit.
Existing Knowledge:
The simplest form of current limiter is a fuse. As the current exceeds the fuse's limits it blows thereby disconnecting the load from the source. This method is most commonly used for protecting house-hold and industrial power supply lines. A circuit breaker is another device used for current limiting. Compared to circuit breakers, fuses attain faster current limitation but their drawback however is that once a fuse is blown, it has to be replaced with a fresh fuse of the correct rating.
Electrical load like AC motors, lighting ballasts and the like can develop extremely high peak inrush currents as soon as the power supply is turned on. Without protection, the only limits on the amount of inrush current drawn are the line impedance, input rectifier drop, and capacitor equivalent series resistance.
High inrush current can affect electrical systems by blowing fuses and tripping circuit breakers unnecessarily. If inrush current protection is not in place, relays and fuses must be used that are rated higher than any possible inrush current. Inrush current can also cause pitted contacts on switches and relays due to the arcing of the contacts. Inrush current can be as high as 100 times the normal steady state current and normally lasts for less than ½ a normal 60 hertz cycle.
Inrush current protection can be provided by an active circuit that uses a combination of power resistors, thyristors, and triacs (A triac or triode for alternating current is an electronic component approximately equivalent to two silicon-controlled rectifiers/thyristors joined parallel but with the polarity reversed and with their gates connected together. This results in a bidirectional electronic switch which can conduct current in either direction when it is triggered). Active circuits are generally expensive and difficult to design. Another option for inrush current protection is a negative temperature coefficient (NTC) thermistor.
NTC-Thermistors are made from various metal oxides that are combined into a powdery mass and mixed with a plastic binding agent. NTC Thermistors at room temperatures offer high initial resistance to the inrush current. Due to the current load and subsequent heating the resistance of the thermistor drops to a few percentage of its resistance at room temperature. At turn on, it presents a high resistance to inrush current and quickly removes itself from the circuit allowing the electrical system to behave normally. NTC thermistor based current limiters are suitable only for low current applications and are not suitable for high current power circuits.
Other current limiting devices used in high current systems include thyristors, superconducting current limiters and fixed impedance inductors. Conventional Inductors with ferromagnetic or nonmagnetic cores are cost efficient, sturdy simple and reliable but are fixed impedance devices for a particular frequency and if kept continuously in circuit lead to running voltage drops under normal operations. The higher the value of the inductor impedance the better is the over-current control. But the normal running voltage drop increases proportionally. Switched inductors are not preferred due the switching time of switching devices. Large fixed impedance inductors will also lead to, fault hanging, wherein for high impedance faults the current is too low for protective relays to operate quickly and reliably.
Conventional fixed impedance inductors are built with windings wound on either a magnetic core or nonmagnetic core or a combination of the two. The permeability of the core is substantially constant for the magnetization from low to high saturation flux densities. This leads to a constant inductance value.
When a magnetic field is applied, a flux is forced through the soft ferromagnetic material. The flux is proportional to the magnetic field intensity. The ratio of flux density to the magnetic field intensity remains substantially constant. This ratio is called the permeability of the magnetic material (mu). Upon magnetization the unaligned fields rotate toward alignment in the direction of magnetization and the size of aligned field increases.
The value of mu for isotropic materials is nearly constant from low flux density to peak flux density. Inductors built employing constant permeability magnetic materials will have nearly constant inductance and inductive reactance for low and high currents at the same frequency.
There is thus a need for a current limiting proportional inductance device, which can be continuously in circuit, with very low impedance and voltage drop for normal currents, but responds to over-currents instantaneously, by building up high impedance without switching devices, so that the current limiting action is proportional to the over-current.